Especially when you learn that he was a last-minute substitute as emperor when Hadrian's first choice, Aelius, "became a god" a bit early.
Hadrian had always intended for Marcus Aurelius to rule eventually, and probably chose Aelius because he was a personal favorite and as a consumptive could be counted on to die after a decent interval, hopefully when young Marcus was old enough to rule on his own. Unfortunately, Aelius was in worse shape than Hadrian knew, and when he died, the aging emperor, his own health fading fast had to find a trustworthy alternate. Who better than his loyal advisor, Antoninus ? The choice was doubly ironic. Aelius had died too soon. But Antoninius ,who at 51 was not expected to rule very long, fooled everyone and reigned nearly 25 years, longer than any emperor since Augustus.
Historians have always been annoyed at Pius. His reign was so moderate and his personal life so exemplary that they can't find anything "interesting" to write about. I agree, Heraklia, he has been sorely underrated.
Consider the character of the man. Besides Marcus' glowing assessment add what Hadrian might have said:
" I say old man, here's the Roman world. Would you mind taking care of it for me? That's a good fellow !" And done so with a pefectly straight face. Quite a guy